Here are the postfix  details:
---------------------------------
#:>postconf -n
alias_database = hash:/etc/postfix/aliases
alias_maps = hash:/etc/postfix/aliases
broken_sasl_auth_clients = yes
command_directory = /usr/sbin
config_directory = /etc/postfix
daemon_directory = /usr/libexec/postfix
data_directory = /var/lib/postfix
debug_peer_level = 2
debug_peer_list = gmail.com
header_checks = regexp:/etc/postfix/header_checks
html_directory = /usr/share/doc/postfix-2.5.6-documentation/html
inet_interfaces = all
mail_owner = postfix
mailq_path = /usr/bin/mailq.postfix
manpage_directory = /usr/share/man
mydestination = localhost
myhostname = sub.domain.tld
mynetworks = xx.xx.xx.xx
newaliases_path = /usr/bin/newaliases.postfix
non_smtpd_milters = unix:/var/run/dk-milter/dk.sock
queue_directory = /var/spool/postfix
readme_directory = /usr/share/doc/postfix-2.5.6-documentation/readme
sample_directory = /etc/postfix
sendmail_path = /usr/sbin/sendmail.postfix
setgid_group = postdrop
smtpd_milters = unix:/var/run/dk-milter/dk.sock
smtpd_recipient_restrictions = permit_mynetworks     permit_sasl_authenticated     check_policy_service unix:private/vpm-pfpolicy     reject_unauth_destination
smtpd_sasl_auth_enable = yes
unknown_local_recipient_reject_code = 550
virtual_mailbox_domains = pgsql:/etc/postfix/vpm-domains
virtual_transport = vpm-pftransport

 -------------------------------
Here is the dk-milter details:

#:>cat /etc/sysconfig/dk-milter
# Default values
#
USER="dk-milt"
PORT="local:/var/run/dk-milter/dk.sock"
SIGNING_DOMAIN="domain.tld"
SELECTOR_NAME="dk1"
KEYFILE="/etc/mail/domainkeys/dk_${SIGNING_DOMAIN}.pem"
SIGNER=yes
VERIFIER=yes
CANON=simple
REJECTION="bad=r,dns=t,int=t,no=a,miss=r"
EXTRA_ARGS="-h -l -D"
MILTER_GROUP="mail"

# User configuration
#
#PORT0="inet:10034@localhost"
#SIGNER0=no
#PORT1="inet:10035@localhost"
#VERIFIER1=no
#...
---------------------------------

So I'm having issues with the Domain Keys signing messages.
I've used dk-milter-1.0.2-0.i386.rpm & followed the installation doc properly. Also my DNS settings & records are perfect.
Now, my problem is that when I send mails using webmail from the local user configured through vPostmaster then the mails are getting signed BUT the issue is that when the mails sent from different machines using their applicaitons then the messages are delivered but Not signed. [ I've already added their IP addreses here: mynetworks = xx.xx.xx.xx in main.cf]
What am i Missing?
Is this a postfix issue or a Domain Keys issue ?

---------------------------------
Here are the Gmail headers:

## Mails Signed:
Received-SPF: pass (google.com: domain of xx@xxx.com designates xx.xx.xx.xx as permitted sender) client-ip=xx.xx.xx.xx;
DomainKey-Status: good (test mode)
Authentication-Results: mx.google.com; spf=pass (google.com: domain of xx@xxx.com designates xx.xx.xx.xx as permitted sender) smtp.mail=xx@xxx.com; domainkeys=pass (test mode) header.From=xx@xxx.com
Message-ID: <04e5e968f1477701780046adc9a54e67.squirrel@xx.xx.xx.xx>
X-DomainKeys: Sendmail DomainKeys Filter v1.0.2 xx.xxx.com 884C2160C78
DomainKey-Signature: a=rsa-sha1; s=dk1; d=xxx..com; c=simple; q=dns;
b=a2VI2luMgivi7pYjjXiLD+Wmm9MYNKvfYdS8x3TiFekVNUowGQz/TiJfvmI0Q43TI
8nnedknImUkrONAsijbqw==


## Mails NOT signed:
Received-SPF: pass (google.com: domain of xx@xxx.com designates xx.xx.xx.xx as permitted sender) client-ip=xx.xx.xx.xx;Authentication-Results: mx.google.com; spf=pass (google.com: domain of xx@xxx.com designates xx.xx.xx.xx as permitted sender) smtp.mail=xx@xxx.com
Message-ID: <265003-2200973221109811@mailrelay90.com>
X-DomainKeys: Sendmail DomainKeys Filter v1.0.2 sub.domain.tld C89AC160C7E

---------------------------------